1. Field of the Invention
The present invention relates to a communication terminal for performing interactive audiovisual communication or audio communication with Internet Protocol (IP) telephony over an IP network and relates to a method for controlling such a communication terminal.
2. Description of the Related Art
Image transmission and reception in videophones, including both a telephone set including a handset connected to a base unit with a cord and a cordless telephone set, in most cases, uses a display screen of only a base unit.
Currently, telephone products supporting interactive audiovisual communication are mostly used in circuit switched networks, such as an integrated services digital network (ISDN), and videophone products capable of placing a call over an IP network are not known.
Examples of videophones used over ISDNs previously proposed include the following:
Japanese Patent Laid-Open No. 6-125551 discloses that a cordless videophone system connected to an ISDN line informs a cordless handset of whether an incoming call received by the cordless handset is in a videophone mode or an audio phone mode from a base unit using a radio signal.
Japanese Patent Laid-Open No. 5-022721 discloses that a cordless videophone system connected to an ISDN line sends audio signals and distance signals to a cordless handset, detects the distance between the cordless handset and a base unit, and zooms a camera of the base unit.
These videophone systems capable of performing an interactive audiovisual communication function (videophone function) of a base unit while audio communication is carried out with a handset offer convenience to some extent by informing the handset of a status of an incoming call when a user answers the call with the handset, as in Japanese Patent Laid-Open No. 6-125551, by zooming the camera in accordance with the distance between the handset and the base unit, as in Japanese Patent Laid-Open No. 5-022721, or the like. These systems are adequate in terms of usage over an ISDN.
However, for the system disclosed in Japanese Patent Laid-Open No. 6-125551, although a user is capable of determining whether an incoming call is in a videophone mode or audio phone mode when responding to the incoming call with the handset, the user is not able to switch the call with the handset. Therefore, the user is not able to perform interactive audiovisual communication while answering the call with the handset. The system thus has disadvantages in that the user must call the caller back from the base unit in order to perform interactive audiovisual communication.
On the other hand, for the system disclosed in Japanese Patent Laid-Open No. 5-022721, the zoom position of a camera is capable of being switched in accordance with the distance between the handset and the base unit, but the display is not switched. Therefore, a user must use the display screen of the base unit every time. As a result, the user is required to be present in the proximity of the base unit. Therefore the system has a problem in that the user has a limited range of motion while talking on the telephone with the handset in interactive audiovisual communication.
Additionally, since ISDN does not square with the reality of communication networks today, the number of subscribers who use ISDN lines as telephone lines is in decline.
Nowadays, the number of users of IP telephony, or voice over Internet Protocol (VoIP), which employs IP networks, is increasing tremendously. Since current communications over IP networks are under development, it is necessary for telephone services to have a terminal supporting both an IP network and a circuit switched network. VoIP originally uses the videoconference standard, so that it is capable of performing interactive audiovisual communication including transmission of both an audio stream and a video stream.